Depletion mode devices are normally ON, while enhancement mode devices are normally OFF. Depletion mode III-nitride heterojunction power semiconductor devices such as III-nitride high electron mobility transistors (BEMTs) are well known.
For various reasons, e.g. efficiency and simplicity of operation, enhancement mode III-nitride power semiconductor devices are desirable. To obtain an enhancement mode III-nitride heterojunction type power semiconductor device, the carrier-rich region (referred to as the two-dimensional electron gas or 2-DEG) under the gate must be interrupted, whereby the device is rendered normally OFF when no voltage is applied to the gate electrode.
In Control of Threshold Voltage of AlGaN/GaN BEMT by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode, Yong Cai, Yugang Zhou, Kei May Lau and Kevin Chen, IEEE Transactions on Electron Devices, Vol. 53, No. 9, September 2006 (Cai et al.) it is taught that using a CF4 process fluoride ions can be embedded in the barrier region of a depletion mode device in order to render the same normally OFF.
In practice, however, a depletion mode III-nitride BEMT which has been turned into an enhancement mode device using the CF4 process is unstable. Specifically, after a period of operation, the threshold voltage of such a device tends to shift back to negative. That is, the device reverts back to depletion mode operation.
It is believed that the electric field between the gate electrode and the drain electrode of a III-nitride power semiconductor device is responsible for the shift of the threshold voltage.
To overcome the drawbacks discussed above, according to one aspect of the present invention, the voltage between the gate and the drain is reduced, whereby a stable enhancement mode III-nitride device, which has been fabricated for example, by using the CF4 process is obtained.
According to one embodiment of the present invention, an enhancement mode III-nitride power device fabricated using the CF4 process is provided with a field plate over the heterojunction which is shorted to the source electrode of the device. As a result, the gate to drain voltage “seen” by the gate electrode is reduced, whereby the device is rendered more stable.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.